1. Field of the Invention
The present invention relates to a hydrodynamic bearing device, and more particularly relates to a hydrodynamic bearing device that is favorable as a bearing device in a hard disk driving device.
2. Background Information
Fluid bearings are used today instead of ball bearings in the spindle motors installed in hard disks. Hard disks equipped with these fluid bearings have been used in the past mainly for desktop computers and notebook computers, but as performance has improved in terms of higher capacity and better resistance to vibration and impact, in recent years these fluid bearings have come to be used in applications other than personal computers in order to meet the needs of the end user, such as video players, music players, video/TV integrated televisions, and other such AV equipment.
There is considerable downward pressure on the price of the hard disks used in these devices. The sample applies to an SPM, which is a component used in hard disks, and the cost needs to be reduced while maintaining good performance in terms of higher capacity and better resistance to vibration and impact.
Each SPM manufacturer has its own approach to achieving the cost reductions demanded by hard disk manufacturers. In particular, reducing the working costs and material costs entailed by an SPM affords a significant reduction in SPM cost.
There is a hydrodynamic bearing device in which a dynamic pressure generation groove is formed in the outer peripheral face of the shaft, and a recess (large diameter component) is provided to the inner peripheral face of the sleeve (see, for example, Japanese Utility Model No. 2,534,872). More specifically, the length of the dynamic pressure generation groove in the axial direction is made longer than the portion other than the recess in the sleeve, that is, the convex portion (small diameter component), so the dynamic pressure generation groove is always across from the convex portion of the sleeve even if the sleeve should shift with respect to the shaft in the axial direction.
With another known hydrodynamic bearing device, two herringbone dynamic pressure generation grooves that are aligned in the axial direction are formed in the inner peripheral face of the sleeve, and an introduction groove that is substantially parallel to the rotational axis is formed at both ends of the herringbone grooves, the result being that there is less imbalance in the axial direction of the dynamic pressure produced in the herringbone dynamic pressure generation grooves (see, for example, Japanese Laid-Open Patent Application 2001-74040). More specifically, any deviation in the inversion timing or manufacturing error in the sleeve is absorbed by the introduction grooves, so there tends to be less axial imbalance in the dynamic pressure if the angled portions of the herringbone grooves are worked for the same amount of time.
With the hydrodynamic bearing device discussed in Japanese Utility Model No. 2,534,872, if the shaft should shift in the axial direction with respect to the sleeve, the lengths of the angled portions of the dynamic pressure generation grooves (the distance from the herringbone peaks to the ends, that is, the effective length of the dynamic pressure grooves) end up being different, so the balance in the generated dynamic pressure is lost, and as a result, flow is generated in the oil in the axial direction, which causes oil leakage or excess lift.
Also, with this bearing structure (in which a recess is provided to the inside diameter portion of the sleeve), forming the sleeve by sinter molding, resin molding, or another such working method involving the use of a metal mold can be employed to reduce material costs and working costs. In general, with a working method involving the use of a metal mold, it is difficult to work a recess in the inside diameter portion of the sleeve because of limitations to the mold structure. Thus, to achieve a balance in the effective lengths of the dynamic pressure generation grooves, it is necessary to add a step in which a recess is cut precisely in the ends and the inside diameter portion of the sleeve, and this drives up the cost.
Let us consider a case in which the forming method is changed in the fluid dynamic bearing structure discussed in Japanese Laid-Open Patent Application 2001-74040 (in which a recess is provided to the inside diameter portion of the sleeve). If the sleeve is formed by a working method involving the use of a metal mold, such as sinter molding or resin molding, in order to reduce material costs and working costs, it is difficult to finish the sleeve in a single step with a working method involving the use of a metal mold. That is, a step of forming a recess in the inside diameter portion is required, and this results in higher cost.
It is an object of the present invention to generate the desired dynamic pressure with an inexpensive configuration.